FRANKFURT—Pressure mounted on
Volkswagen AG
on Monday as German prosecutors launched a criminal investigation of former Chief Executive
Martin Winterkorn
and other unnamed executives on suspicion of fraud in connection with an emissions cheating scandal that has rocked the car maker.

The opening of the investigation was widely expected after Mr. Winterkorn resigned under pressure last week amid the worst crisis in VW’s 78-year history. He apologized publicly for what he called unimaginable misconduct at the company, but insisted that he had been unaware of the company’s efforts to dupe emissions testers.

“As CEO I accept responsibility for the irregularities that have been found in diesel engines and have therefore requested the Supervisory Board to agree on terminating my function as CEO of the Volkswagen Group,” Mr. Winterkorn said in a written statement on Sept. 23. “I am doing this in the interests of the company even though I am not aware of any wrongdoing on my part.”

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The probe is looking at Mr. Winterkorn and other members of Volkswagen’s top management, a spokeswoman for the prosecutor’s office said. “We are investigating Winterkorn and other responsible people,” the spokeswoman said. “We are pursuing every possible lead.”

Volkswagen, Europe’s biggest car maker, has been engulfed by an emissions scandal since U.S. environmental authorities revealed on Sept. 18 that the German car maker had intentionally manipulated emissions tests of some of its diesel-powered cars to pass U.S. tests.

The company later admitted that 11 million cars were affected, raising the specter of one of the largest product recalls to ever hit the global auto industry.

In the days since the initial revelation, VW has become the subject of a criminal investigation by the U.S. Department of Justice and is being investigated by several environmental regulators in the U.S., Europe and Asia. Sales of VW models affected by the software have been halted in some countries.

On Friday, the company appointed
Matthias Müller,
head of its sports-car maker Porsche, to succeed Mr. Winterkorn. Mr. Müller pledged a swift and thorough investigation. The company has suspended three senior engineers pending a fuller investigation to determine whether they were involved in duping environmental regulators.